1. Field of the Invention
The invention relates to a technique for recovering valuable resources from a secondary battery. In particular, the invention relates to a technique for stripping and recovering an active material or constituents thereof from an electrode of a secondary battery (e.g., a lithium secondary battery).
2. Description of the Related Art
Many valuable metals that are precious as resources are used as active materials of a secondary battery, and emphasis has been given to the recovery of the metals from used batteries and the reutilization of the metals. For example, lithium cobalt oxide has recently been in heavy usage as a positive active material for lithium-ion secondary batteries, and there are demands for a technique for recovering a positive active material containing high-value cobalt with high purity. As a prerequisite for recovering this active material with high purity, it is necessary to reliably separate the active material from the current collector of an electrode. However, an electrode for lithium-ion secondary batteries has a structure in which the active material is securely attached to the surface of a current collector and applied thereto as a thin film to improve durability against the multiple charge/discharge cycles and to prevent the electrode from being stripped even when in contact with an organic solvent of an electrolytic solution. For this reason, the current collector and the active material are not easily separated from each other through a physical operation.
In view of the difficulty in separating the active material from the current collector a stripping agent is generally used. In particular, a method uses various stripping agents are used on a waste electrode that includes an active material and the active material is thereby stripped from the waste electrode and recovered. A method of stripping and recovering an active material from a waste electrode in this manner is described in each of the following documents of the related art. For example, Japanese Patent Application Publication No. 9-195071 (JP-A-9-195071) describes a method in which a metal foil-applied waste material of a secondary battery is exposed to a stripping agent made of an organic solution mainly constituted of alkyl phosphoric acid or dialkyl phosphoric acid to strip a metal foil (current collector) of the metal foil-applied waste material and an electrode material containing an active material (lithium cobalt oxide or the like) from each other. According to this stripping method, the active material in the metal foil-applied waste material and the metal foil are stripped directly from each other, and hence can be separated easily from each other and recovered. Japanese Patent Application Publication No. 10-255862 (JP-A-10-255862) describes cases where an acid solution (hydrofluoric acid, sulfuric acid), an alkali metal hydroxide solution (sodium hydroxide), and the like are used as stripping agents instead of an organic solvent. This publication describes that the use of these stripping agents makes it possible to separate an active material and a metal foil (aluminum foil) from each other without using an organic solvent, which is onerous to handle.
However, when an organic solution of alkyl phosphoric acid as described in Japanese Patent Application Publication No. 9-195071 (JP-A-9-195071) is used as a stripping agent, it takes a relatively long treatment time (10 to 180 minutes) to separate an active material and a metal foil from each other. Therefore, high efficiency is not expected. In addition, a water-insoluble organic solvent medium is used as a solvent for dissolving alkyl phosphoric acid. Therefore, from the standpoint of environment conservation, the organic solvent medium needs to be recovered after a stripping treatment. Furthermore, attention needs to be paid to the flammability of the organic solvent medium as well. Thus, the operation of stripping is bothersome, and also, the cost of recovery may be comparatively high. On the other hand, according to a method in which an active material and a metal foil current collector (aluminum foil) are separated from each other through immersion in an acid solution such as sulfuric acid or the like, part of the aluminum foil dissolves. Therefore, the amount of impurities mixed in with the stripped and recovered active material increases, and moreover, metal constituents turned into a solution also need to be recovered separately. Further, when an active material and an aluminum foil are separated from each other through immersion in an alkali metal hydroxide solution, the aluminum foil, which is an amphoteric substance, corrodes to generate a large amount of aluminum hydroxides. Therefore, there arises an additional need to perform a treatment of separating aluminum. When impurities get mixed in with an active material stripped and recovered as described above, a refinement treatment process and a post-treatment, which are troublesome and time-consuming, need to be performed. Thus, a rise in cost is caused, and therefore, there may arise circumstances that make it difficult to reutilize the active material for batteries.
Document D5 (U.S. Pat. No. 4,650,553) discloses that lead is recovered from lead-containing scrap metal in an electrolytic process employing an aqueous solution of C1-4 alkanesulfonic acid as the electrolyte at high concentration, scrap lead as the anode, and an electro-conductive cathode, impressing an electromotive force across the solution between the electrodes to provide a specified steady-state concentration of lead salt in the electrolyte, and continuing the process to deplete the anode and collect lead at the cathode.
Document D6 (U.S. Pat. No. 5,520,794) discloses an electrowinning process for lead incorporating a variety of alkanesulfonic acid-based electrolytes in the absence of a redox couple; an inert anode and a lead acceptable cathode are utilized.
In each of the above described methods, there is still room for improvement in reducing the time required for stripping and recovering, the purity of the stripped and recovered active material, the cost of recovery, and the like.